On Tue, May 06, 2014 at 01:14:26PM +0200, Hendrik Siedelmann wrote: > On 06.05.2014 12:59, Hugo Mills wrote: > >On Tue, May 06, 2014 at 12:41:38PM +0200, Hendrik Siedelmann wrote: > >>Hello all! > >> > >>I would like to use btrfs (or anyting else actually) to maximize raid0 > >>performance. Basically I have a relatively constant stream of data that > >>simply has to be written out to disk. So my question is, how is the block > >>allocator deciding on which device to write, can this decision be dynamic > >>and could it incorporate timing/troughput decisions? I'm willing to write > >>code, I just have no clue as to how this works right now. I read somewhere > >>that the decision is based on free space, is this still true? > > > > For (current) RAID-0 allocation, the block group allocator will use > >as many chunks as there are devices with free space (down to a minimum > >of 2). Data is then striped across those chunks in 64 KiB stripes. > >Thus, the first block group will be N GiB of usable space, striped > >across N devices. > > So do I understand this correctly that (assuming we have enough space) data > will be spread equally between the disks independend of write speeds? So one > slow device would slow down the whole raid?
Yes. Exactly the same as it would be with DM RAID-0 on the same configuration. There's not a lot we can do about that at this point. > > There's a second level of allocation (which I haven't looked at at > >all), which is how the FS decides where to put data within the > >allocated block groups. I think it will almost certainly be beneficial > >in your case to use prealloc extents, which will turn your continuous > >write into large contiguous sections of striping. > > Why does prealloc change anything? For me latency does not matter, only > continuous troughput! It makes the extent allocation algorithm much simpler, because it can then allocate in larger chunks and do more linear writes > > I would recommend thoroughly benchmarking your application with the > >FS first though, just to see how it's going to behave for you. > > > > Hugo. > > > > Of course - it's just that I do not yet have the hardware, but I plan to > test with a small model - I just try to find out how it actually works > first, so I know what look out for. Good luck. :) Hugo. -- === Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk === PGP key: 65E74AC0 from wwwkeys.eu.pgp.net or http://www.carfax.org.uk --- "I am the author. You are the audience. I outrank you!" ---
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